The MP3 codec doesn't work particularly well with lower bitrates. It also will vary greatly depending on the settings used to encode it. A 192kbps variable bitrate track can sound better and be smaller than a 128kbps bitrate track depending on the source material.
The 3 stations you listed in the original post, WPDH and WZZO both are using MPEG AAC (mp4a), while the FM Rock (at 128kbps) is using MP3.
